1. Field of the Invention
This invention relates to methods for producing n-type semiconducting diamond, particularly such methods which utilize a chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) preparation of carbon to form a diamond structure wherein a deposition substrate is heated by a rhenium hot wire, and more particularly to semiconducting diamond materials having a uniformly distributed concentration of rhenium throughout.
An n-type semiconducting diamond is indispensable to the fabrication of semiconducting diamond devices which depend on a p-n junction for their successful operation. Several attempts have been made to dope diamond with impurities by the super-high pressure synthesis process or ion implantation but none has succeeded in synthesizing an n-type semiconducting diamond having low electrical resistivity and high electron mobility.
The present invention was developed to fill a need for a semiconducting diamond material having n-type conductivity. Semiconducting diamond has numerous attributes which are attractive for high-frequency, high-power semiconductor applications. These properties include a high electrical field breakdown voltage, elevated temperature stability, high electron and hole mobilities, high thermal conductivity, and excellent resistance to radiation.
2. Background Art
Semiconducting synthetic or natural diamonds are mostly prepared or found as p-type materials, with boron atoms being the most common impurity. M. W. Geiss. D. D. Rathman, D. J. Ehrlich. R. A. Murphy, W. T. Lindley, xe2x80x9cHigh-Temperature Point-Contact Transistors and Schottky Diodes Formed on Synthetic Boron-Doped Diamond,xe2x80x9d IEEE Electron Device Letters, Vol. EDL-8, No. 8, (August 1987), pp. 341-343, discloses the formation of point-contact transistors on synthetic boron-doped diamond. The diamond used in formation of the point-contact transistor is a diamond single crystal produced using a high-temperature-high-pressure process. H. Shiomi, Y. Nishibayashi, N. Fujimore, xe2x80x9cElectrical Characteristics of Metal Contacts to Boron-Doped Diamond Epitaxial Films,xe2x80x9d Japanese Journal of Applied Physics, Vol. 28, No. 5, (May 1989), pp.758-762, teaches the production of a planar field effect transistor device based on a diamond film. The device disclosed comprises a single crystal diamond substrate on which a single crystal epitaxial layer of boron doped diamond is deposited, thereby producing a p-type semiconductor layer. Titanium (Ti) source and drain contacts, as well as an aluminum (Al) gate Schottky contact are deposited on the diamond film. Type IIb diamonds are described, in A. S. Vishnevskil, A. G. Gontar, xe2x80x9cElectrical Conductivity of Heavily Doped P-Type Diamond,xe2x80x9d Soviet Physics-Semiconductor, Vol. 15(6), (June 1981), pp. 659-661; A. S. Vishnevskil, A. G. Gontar, xe2x80x9cElectrical Conductivity of Synthetic Diamond Crystals,xe2x80x9d Soviet Physics-Semiconductor, Vol. 11(1), (October 1977), pp.1186-1187; G. N. Bezrukov, L. S. Smrnimov, xe2x80x9cSome Electrical and Optical Properties of Synthetic Semiconducting Diamonds Doped With Boron,xe2x80x9d Soviet Physics-Semiconductor, Vol. 4(4), (October 1970), pp. 587-590; J. J. Hauser, J. R. Patel, xe2x80x9cHopping Conductivity in C-Implanted Amorphous Diamond, or How to Ruin a Perfectly Good Diamond,xe2x80x9d Solid State Communications, Vol. 18, (1976), pp. 789-790; I. G. Austin, R. Wolfe, xe2x80x9cElectrical and Optical Properties of a Semiconducting Diamond,xe2x80x9d Proc. Phys. Soc., (1956), pp. 329-338; P. T. Wedepohl, xe2x80x9cElectrical and Optical Properties of Type IIb Diamonds,xe2x80x9d Proc. Phys. Soc., Vol. LXX, No. 2B, (1957), pp.177-185; A. T. Collins, A. W. S. Williams, xe2x80x9cThe Nature of the Acceptor Centre in Semiconducting Diamond,xe2x80x9d J. Phys. C: Solid St. Phys., Vol. 4, (1971), pp. 1789-1800; and V. S. Vavilov, xe2x80x9cIon Implantation into Diamond,xe2x80x9d Radiation Effects, Vol. 37, (1978), pp. 229-236.
Impurities such as lithium which produce n-type diamond have been incorporated into previously formed diamond crystal lattices by ion implantation methods. See U.S. Pat. No. 5,792,256 to Kucherov, et al., and xe2x80x9cElectrical Properties of Diamond Doped by Implantation of Lithium Ions,xe2x80x9d V. S. Vavilov, E. A. Konorova, E. B. Stepanova, and E. M. Trukhan, Soviet Physics-Semiconductors, Vol. 13(6), (1979), pp.635-638. In the case of the former employs a nuclear transmutation technique to convert boron-10, previously incorporated into a carbon-12 matrix, to lithium-7 by bombardment with a high flux of thermal neutrons. The matrix is then converted to a diamond lattice by initiating a solid phase transformation in the carbon using high pressure and temperature.
The latter method produces thin layers of n-type diamond having lithium directly inserted by high energy implantation. The thickness of the layer is, therefore, limited by the energy of accelerated ions that are not infinite. Moreover, the incorporation of n-type impurities into diamond crystal lattices by ion implantation causes a severely damaged surface layer due to graphitization of the diamond which cannot be removed by annealing. Doping with lithium by this method to concentrations higher than 1015/cm3 also leads to graphitization. The implanted crystal must then be heat treated to electronically activate the implanted impurity. The severely damaged layer resulting from ion implantation and the inhomogeneity and significant concentration gradients of the implanted ion across the implanted film thickness render the n-type diamond unsuitable for semiconductor applications.
Another method for producing n-type diamond involves the formation of p-type diamond thin film by adding a boron compound such as diborane to the raw material gas for microwave plasma chemical vapor deposition and then converting boron-10 to lithium-7 by neutron irradiation. However, the chemical vapor deposition method limits the depth of the semiconducting substrate to approximately 100 microns. Moreover, the material is unsuitable for semiconductor materials because of radioactive isotopes created during neutron irradiation.
Other attempts to make an n-type semiconducting diamond are described in V. S. Vavilov, E. A. Konorova, xe2x80x9cElectric Properties of Diamond Doped by Implantation of Lithiumxe2x80x9d, Soviet Physics-Semiconductors, Vol. 13(6), p. 635 (1979); in V. S. Vavilov, E. A. Konorova, xe2x80x9cConductivity of Diamond Doped by Implantation of Phosphorus Ions,xe2x80x9d Soviet Physics-Semiconductors, Vol. 9(8), (1976), pp. 962-964; in V. S. Vavilov, E. A. Konorova, xe2x80x9cImplantation of Antimony Ions Into Diamond,xe2x80x9d Soviet Physics-Semiconductors, Vol. 6(12), (1972), pp. 1998-2002; in Jean-Francois Morhange, xe2x80x9cStudy of Defects Introduced by Ion Implantation in Diamond,xe2x80x9d Japanese Journal of Applied Physics, Vol. 14(4), (1975), pp. 544-548; in Tsai et al, xe2x80x9cDiamond MESFET Using Ultrashallow RTP Boron Doping,xe2x80x9d IEEE Electron Device Letters, Vol. 12, No. 4, (April 1991), pp. 157-159; and in Gildenblat, et al., xe2x80x9cHigh-Temperature Thin-Film Diamond Field-Effect Transistor Fabricated Using a Selective Growth Method,xe2x80x9d IEEE Electron Device Letters, Vol. 12, No. 2, (February 1991), pp. 37-39.
Finally, the art closest to the present invention is discussed in U.S. Pat. No. 5,051,785 to Beetz, Jr. et al., and U.S. Pat. No. 5,381,755 to Glesener, et al. In the former, a chemical vapor method for producing an n-type diamond semiconductor is described, wherein lithium, antimony, bismuth, arsenic, phosphorous, and scandium are used as doping ions. A chemical vapor deposition method is used to laid-down the carbon film. A methane/hydrogen gas mixture is flowed over a hot tungsten filament to heat and dissociate the carbon-containing gas. Beetz""s n-type impurity, however, is introduced with the gas in order that the impurity be incorporated into the carbon lattice in situ. Beetz, Jr., et al., however, did not recognize that the hot filament itself could act as a source of n-type impurity material. The latter patent describes a method for growing a doped diamond film wherein a diamond carbon layer is deposited from a carbon containing gas mixture. As with Beetz, et al., however, Glesener, et al., does not recognize the efficacy of doping with a refractory filament wire.
Therefore, there remains a need to provide a more satisfactory solution for producing a robust, bulk diamond semiconductor material exhibiting n-type conductivity.
The present invention seeks to resolve at least some of the problems which have been experienced in the background art, as identified above.
Specifically, it is an object of this invention to provide an n-type semiconducting diamond and a process for producing the same.
It is still another object of the invention is to obtain an n-type semiconducting diamond rather than an implanted thin layer.
It has now been found that the above objects of the present invention can be attained by a semiconducting diamond comprising diamond carbon and rhenium.
A further object of the present invention is to obtain n-type semiconducting diamond having a uniformly distributed concentration of rhenium throughout the diamond lattice.
It is yet another object of this invention to provide a method for providing an n-type semiconducting diamond material.
Those skill in the art will appreciate that the objects of this invention constitutes an important advance in the art of n-type semiconducting diamond, as evidenced by the foregoing objects. Additional objects and advantages of the invention over the background art will become apparent from the description which follows, or may be realized by practicing the invention.